Question

(a) Methane \(\left(\mathrm{CH}_{4}\right)\) and the perchlorate ion \(\left(\mathrm{ClO}_{4}^{-}\right)\) are both described as tetrahedral. What does this indicate about their bond angles? (b) The \(\mathrm{NH}_{3}\) molecule is trigonal pyramidal, while \(\mathrm{BF}_{3}\) is trigonal planar. Which of these molecules is flat?

Short answer

(a) The bond angles in both methane (CH₄) and the perchlorate ion (ClO₄⁻) are 109.5°, because they have a tetrahedral geometry. (b) The BF₃ molecule is flat, while the NH₃ molecule is not, as BF₃ has a trigonal planar geometry and NH₃ has a trigonal pyramidal geometry due to the presence of a lone pair of electrons.

Step-by-step solution

Tetrahedral Geometry

Both CH₄ and ClO₄⁻ are described as having tetrahedral molecular geometries. This means that they have central atoms (C in CH₄ and Cl in ClO₄⁻) surrounded by four bonded atoms, all positioned at the vertices of a regular tetrahedron.

Tetrahedral Bond Angles

The bond angles of a tetrahedral molecule are always 109.5°. Since both CH₄ and ClO₄⁻ are tetrahedral, the bond angles in both of these molecules will be 109.5°. Answer to (a): The bond angles in both methane (CH₄) and the perchlorate ion (ClO₄⁻) are 109.5°.

Understanding Trigonal Pyramidal and Trigonal Planar Geometry

NH₃ has a trigonal pyramidal geometry, which means it has a central atom (N) surrounded by three bonded atoms (H) and a lone pair of electrons. The atoms are positioned at the vertices of a triangle, while the lone pair occupies the position above the central atom, forming a pyramid shape. BF₃, on the other hand, has a trigonal planar geometry - it has a central atom (B) surrounded by three bonded atoms (F) arranged at the vertices of an equilateral triangle, with all atoms lying in the same plane.

Determine Flat Molecules

From the geometries described in Step 3, we can see that NH₃ is not flat because it has a pyramidal shape resulting from the presence of the lone pair of electrons. Conversely, BF₃ is considered flat because the central atom and all the bonded atoms lie in the same plane. Answer to (b): The BF₃ molecule is flat, while the NH₃ molecule is not.